Fuel cells stack temperature variations

Table 8-2. Fuel cell stack temperature variations as function of operating and cooling conditions...

The generation of heat always accompanies the operation of a fuel cell. The heat is due to inefficiencies in the basic fuel-cell electrochemical reaction, crossover (residual diffusion through the fuel-cell solid-electrolyte membrane) of fuel, and electrical heating of interconnection resistances. Spatial temperature variation can occur if any of these heat-generating processes occur preferentially in different parts of the fuel cell stack. For example, non-uniform distribution of fuel across the surfaces of electrodes, different resistances between the interconnections in a stack, and variations among... 

Temperature Distribution Along the fuel cell channel, the temperature distribution is directly controlled by the heat transfer boundary conditions. For small fuel cell stacks, with no active coolant flow, the external boundary conditions control the temperature distribution and at low current can be considered as uiriform in temperature. For larger stacks with active cooling, the temperature distribution can be engineered to match the desired humidity profile to control flooding and promote longevity by ehmination of dry- and hot-spot locations. In the in-plane direction, temperature variation exists, with more water accumulation under generally colder lands, as discussed. The temperature distribution in the stack can be fairly... 

Equation (6-45) may be solved numerically for a variety of boundary conditions, such as constant T at the walls, or constant or prescribed beat flux. Because of complicated three-dimensional heat transfer pathways (shown in Figure 6-23), calculation of heat fluxes and temperature profiles in a fuel cell stack requires 3-D numerical simulation. Figure 6-25 shows temperature distribution in a representative cross-section of a fuel cell obtained by 3-D numerical simulation [28]. From Figure 6-25, it is obvious that there are significant temperature variations inside a fuel cell stack. Because most heat in a fuel cell stack is produced in the cathode catalyst layer, that layer expectedly has the highest temperature. 

All PAFC stacks are fitted with manifolds that are nsnally attached to the ontside of the stacks external manifolds) We shall see later that an alternative internal manifold arrangement is preferred by some MCFC developers. Inlet and outlet manifolds simply enable fnel gas and oxidant to be fed to each cell of a particnlar stack. Carefnl design of inlet fnel manifold enables the fuel gas to be supplied uniformly to each cell. This is beneficial in minimising temperature variations within the stack thereby ensuring long lifetimes. Often a stack is made of several sub-stacks arranged with the plates horizontal. 

Additionally, we seek diagnostic tools to understand how the fuel cell performance varies with the location in an individual fuel cell and between fuel cells in a stack. Spatial and cell-to-cell variations in current, temperature, reactant concenhation, and other parameters occur, especially at moderate to high currents and during load transients, and tools are needed which can measure or directly observe these effects. In an operating stack, the number of sensors are Umited due to various cost and size constraints, but laboratory diagnostics are very sophisticated. To understand distributed effects such as flooding in PEFCs or temperature distribution in SOFCs, direct visualization tools and sensors are... 

Figures 5.22 and 5.23 show the y-currcnt density and temperature distribution respectively in the central x -section and various z-sections of the co-flow stack. The air and fuel in this case are both entering from left (z = 0 m). It can be seen from Figure 5.22 that the current densities are the highest in the z-planes near the gas inlets and they gradually decrease in the subsequent planes. The cell to cell variations in current density distribution are not obvious in Figure 5.22 but the profiles of y-current density along the z-direction taken at the center of each cell, plotted in Figure 5.24a show that there is maximum variation between the first two cells with the rest more or less being the same. It is not clear why such a large difference is observed between the first two cells. Perhaps this might be an artifact of the type of boundary conditions imposed at the bottom and the top surfaces.

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